Improved thermodynamic model for interaction of EDTA with trivalent actinides and lanthanide to ionic strength of 6.60 m

Abstract

The dissociation constants of ethylenediaminetetraacetic acid (H4EDTA), and the stability constants of Am3+, Cm3+and Eu3+ with EDTA4− have been determined at 25°C, over a range of concentration varying from 0.1 to 6.60m NaClO4 using potentiometric titration and an extraction technique, respectively. The formation of only 1:1 complex, M(EDTA)−, where (M=Am3+, Cm3+ and Eu3+), was observed under the experimental conditions. The observed ionic strength dependencies of the dissociation constants and the stability constants have been described successfully over the entire ionic strength range using the Pitzer model. The thermodynamic stability constant: logβ1010=20.55±0.18 for Am3+, logβ1010=20.43±0.20 for Cm3+ and logβ1010=20.65±0.19 for Eu3+ were calculated by extrapolation of data to zero ionic strength in an NaClO4 medium. In addition, logβ1010 of 20.05±0.40 for Am3+ was obtained by simultaneously modeling data both in NaCl and NaClO4 media. For all stability constants, the Pitzer model gives an excellent representation of the data using interaction parameters β(0), β(1), and Cϕ determined in this work. The improved model presented in this work would enable researchers to model accurately the potential mobility of actinides (III) and light rare earth elements to ionic strength of 6.60m in low temperature environments in the presence of EDTA.